ABSTRACT The objective of this study was to determine the optimum age at last weighing and compare the goodness of fit of nonlinear models used to fit longitudinal weight-age data to describe the growth pattern of Anglo-Nubian does. Weights of 104 animals from birth to 60 months of age were grouped into 10 age groups at six-month intervals. In each age group, parameters A (asymptotic weight), B (integration constant), and K (maturity index) were estimated using the Brody, Gompertz, logistic, and von Bertalanffy models. Data were analyzed using analysis of variance in a factorial design (10 age groups × 4 nonlinear models). The age group × model interaction was not significant. Mean estimates of A, B, and K were significantly different between age groups up to 30 months (p < 0.05), indicating that the estimated curve is affected by weights taken before this age independent of the model. The values of mean squared error (MSE), mean absolute deviation (MAD), coefficient of determination (R2) and Rate of convergence (RC) at each age group up to 30 months were compared to determine the goodness of fit of nonlinear models. The ranking of fit was logistic, Brody, von Bertalanffy, and Gompertz. The logistic and Brody models respectively estimated the smallest and largest asymptotic weight. Longitudinal weight records taken until 30 months of age are most appropriate for estimating the growth of Anglo-Nubian does using nonlinear models. | RESUMO Objetivou-se avaliar a idade da pesagem e a qualidade do ajuste de modelos não lineares a dados longitudinais para descrever o padrão de crescimento de fêmeas Anglonubiana. Pesos de 104 animais do nascimento até 60 meses de idade, foram agrupados em 10 classes, com intervalos de seis meses. Em cada classe estimou-se parâmetros A, B e K com modelos Brody, Gompertz, Logístico e Von Bertalanffy. Realizou-se análise de variância com fatorial (10 classes x 4 modelos) com 104 repetições, considerando-se os três parâmetros como características. Baseou-se na significância da média dos parâmetros nas classes para identificar a melhor idade de pesagem. Não ocorreu interação "classe-modelos". As médias dos três parâmetros nas classes com pesos registrados até 30 meses diferiram entre si (P<0,05), indicando influência de pesagens inferior a essa idade na curva estimada, independente do modelo. Para identificar modelos com melhor ajuste aos dados, utilizou-se os valores do QMR, DMA, R2 e RC que cada um apresentou na classe com pesagens até 30 meses. Constatou-se como sequência de melhor ajuste: Logístico, Brody, Von Bertalanffy e Gompertz. O modelo Logístico estima menor peso assintótico e Brody o maior. Pesagens até 30 meses de idade se adequam à análise do padrão de crescimento de cabras com modelos não lineares.

The objective of this study was to determine the optimum age at last weighing and compare the goodness of fit of nonlinear models used to fit longitudinal weight-age data to describe the growth pattern of Anglo-Nubian does. Weights of 104 animals from birth to 60 months of age were grouped into 10 age groups at six-month intervals. In each age group, parameters A (asymptotic weight), B (integration constant), and K (maturity index) were estimated using the Brody, Gompertz, logistic, and von Bertalanffy models. Data were analyzed using analysis of variance in a factorial design (10 age groups × 4 nonlinear models). The age group × model interaction was not significant. Mean estimates of A, B, and K were significantly different between age groups up to 30 months (p < 0.05), indicating that the estimated curve is affected by weights taken before this age independent of the model. The values of mean squared error (MSE), mean absolute deviation (MAD), coefficient of determination (R2) and Rate of convergence (RC) at each age group up to 30 months were compared to determine the goodness of fit of nonlinear models. The ranking of fit was logistic, Brody, von Bertalanffy, and Gompertz. The logistic and Brody models respectively estimated the smallest and largest asymptotic weight. Longitudinal weight records taken until 30 months of age are most appropriate for estimating the growth of Anglo-Nubian does using nonlinear models.